netdev - Re: [PATCH v9] tilegx network driver: initial support

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Date:	Wed, 06 Jun 2012 20:10:23 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	bhutchings@...arflare.com, arnd@...db.de,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v9] tilegx network driver: initial support

On Mon, 2012-06-04 at 16:12 -0400, Chris Metcalf wrote:
> This change adds support for the tilegx network driver based on the
> GXIO IORPC support in the tilegx software stack, using the on-chip
> mPIPE packet processing engine.
> 

> +
> +/* Do "TSO" handling for egress.
> + *
> + * Normally drivers set NETIF_F_TSO only to support hardware TSO;
> + * otherwise the stack uses scatter-gather to implement GSO in software.
> + * On our testing, enabling GSO support (via NETIF_F_SG) drops network
> + * performance down to around 7.5 Gbps on the 10G interfaces, although
> + * also dropping cpu utilization way down, to under 8%.  But
> + * implementing "TSO" in the driver brings performance back up to line
> + * rate, while dropping cpu usage even further, to less than 4%.  In
> + * practice, profiling of GSO shows that skb_segment() is what causes
> + * the performance overheads; we benefit in the driver from using
> + * preallocated memory to duplicate the TCP/IP headers.
> + */

All this stuff cost about 300 lines of code in this driver, without IPv6
support.

I am pretty sure this performance problem should be solved in net/{core|
ipv4|ipv6} instead

What TCP performance do you get with TSO/GSO and SG off ?



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